GB2168201A - Transformer cores - Google Patents

Abstract

High power transformer cores are made of laminations formed of steel strip which usually comes covered by an insulating coating providing a tensile stress on the steel in the rolling direction. Such a coating gives a minimum power loss when in the direction of magnetisation in the core. However at the joints and corners, where the direction of magnetisation is not in the rolling direction, it has been found that a higher power loss is achieved. Therefore, the tensile coating is removed from the joint regions 18, 19, and a compressive coating may be applied in its place. Alternatively or additionally the joint regions may be scribed to reduce the effect of the coating. <IMAGE>

Description

SPECIFICATION Transformer cores The present invention relates to cores for transformers and particularly to laminated cores for large power transformers.

Usually, a grain-oriented electromagnetic steel sheet is employed for the construction of such cores. The grain-orientated electromagnetic steel sheet is comprised of crystal grains which have a so called Goss texture and which have an (110)[001] orientation expressed by Miller indices. This designation indicates that the (110) plane of the crystal grains are parallel to the sheet surface, while the [001] axis of the crystal grains, i.e. the direction of easy magnetisation, is parallel to the rolling direction. Thus, the magnetic properties of the grain-orientated eletromagnetic steel sheet are excellent in the rolling direction and deteriorate with a deviation of angle of magnetisation from the rolling direction.The grain-orientated electromagnetic sheet steel is, therefore cut or blanked in such a manner that the direction of magnetic flux through the core of the transformer is coincident with the rolling direction.

This is shown in Fig. 1(a) where the coiled sheet steel 1 is unwound with the rolling direction shown by the arrow A and a blank 2 of the limbs of a transformer core 3 is cut out with the major lineal dimension along the rolling direction. As shown in Fig. 1(b) the core is composed of several legs 4 and yokes 5 each made up of laminations stacked together and overlapping at the joint regions. The direction of rolling is shown by the arrows B in Fig. 1(b) and the magnetic flux path is shown by arrows C in Fig. 1(c) which also shows the overlapping regions of the laminations at the joints by showing the next succeeding layer in dotted outline.

The grain-orientated electrical steel has special magnetic properties which make it amenable for use in transformer cores, since it demonstrates lower values of specific power loss per unit weight of core than other types of electrical steel. The steel has very close alignment of the preferential easily magnetised direction of the polycrystalline structure along the rolling direction of the steel strip 7 as shown in Fig. 2. As a consequence of the method of producing this type of steel, the mean grain size of the polycrystalline matrix 6 is exceedingly large, some 1-2 cm in diameter, and this can offset the lower power loss benefits obtained from the improved alignment of the polycrystalline structure.

The fundamental phenomenon which relates grain size, orientation, state of tensile strain and other material variables, with the observed low values of total power loss under cyclic magnetisation is the magnetic domain structure of the electrical steel material. This is illustrated in Fig. 2(a) where within each single crystalline 8 of the polycrystalline matrix 6 the magnetic moment vectors of individual atoms align up over comparatively large distances such that in typical grains of this material there are well defined magnetically aligned regions called domains 9, which are separated by regions of transition of the magnetic direction termed domain walls 11, and which extend completely through the material thickness. With well aligned crystals the pattern demonstrated by the domains is simple.

They take the shape of rectangular parellelpipeds and adjacent domain values have directions of magnetisation 10 which are opposite to each other. As the size of the grains is increased the width of these main magnetic domains also increases. Large magnetic domains are not, however, conducive to low values of power loss because on applying an external magnetic field, the domains with magnetisation in the same direction as that of the external field expand at the expense of the other domains. Thus, with large domains with a corresponding small number of domain walls 11, the domain walls move further and further to accommodate a magnetic field increment than the case where there are small domains and a large number of domain walls. In the former case the higher velocity of the domain walls gives rise to a higher value of the eddy current component of no-load loss.

As illustrated in Fig. 2(b), however, on grains mis-orientated from the (1 10)[001], the main domains also have a surface structure of domains called closure domains 12 which are formed to minimise the total magnetic energy of the composite domain structure. The closure domains are aligned at an angle 4s to the rolling direction to form a dentritic shaped pattern. When the electrical steel is magnetised along the rolling direction, (as in limbs and yokes), the predominant process is one or rearrangement of the main domains, as explained above, at the expense of the closure domains.However when the steel is magnetised at angles to the rolling direction (as in the core joint regions), the magnetisation process is accomplished by firstly nucleation and subsequently, rearrangement of the closure domain structure at the expense of the main domain structure. High levels of magnetic field are required to nucleate sufficient closure domains to initiate magnetisation of the steel strip/core by rearrangement of the closure domain structure.

Clearly, therefore, to minimise the power loss when the steel is magnetised in the rolling direction, it is desirable to increase the number of main domains so that they are fairly small but aligned with the direction of magnetisation. Similarly, to minimise the power loss when the steel is magnetised at angles to the rolling direction, it is desirable to increase the number of closure domains.

One known method for increasing the number of main domains and thus decreasing the power loss when the steel is magnetised in the rolling direction is to apply a high-stress insulative coating onto the finished grain-orientated electromagnetic steel as taught in U.S.

Patent No. 3996073. Such coatings place the grain-orientated steel strip in tensiqn in the rolling direction of the steel which causes a decrease in the width of the main domains and a reduction in the number of closure domains. Many commercially available grain-orientated electromagnetic steel sheets have such a coating.

Mechanical, or physical, scribing transverse to the sheet rolling direction is another technique that has been found to be effective in reducing the main domain spacing and lowering the power losses. This is because scribing lines across the sheet has the effect of nucleating new domains by creating additional domain walls which subdivide the existing domains and thus increase the number of such main domains. The disadvantages of mechanical scribing are, however, that the insulative coating is damaged and the surface of the steel sheet becomes uneven. These factors tend to produce increased interlaminar losses in a transformer manufactured from a steel so treated.

It has also been proposed to irradiate the grain-orientated steel strip with a laser beam.

The steel is irradiated at or nearly transverse to the direction of rolling and this induces a domain substructure by nucleating new domains in the same way as described above and thus will henceforth be called laser scribing. The scribing can take place in such a way as to mark the steel sheet and thus ablate the insulative coating (see European Patent Nos.

0008385, 0033878 and 0087587) or in such a way that the steel is not marked and the coating is not damaged (see European Patents Nos. 0100038 and 0102732).

In U.K. Patent No. 2062972 there is disclosed a method of producing a core for electrical machinery, such as transformers, which has been laser-scribed in a direction either transverse or parallel to the rolling direction of the steel so as to produce decreased power loss. In all cases, the elongated portions of the core are scribed in only these two directions and the joint regions may or may not be so scribed.

However, during operation of the transformer, magnetic flux traverses around the core and at the joint regions, that is at the corner joints and the Joints, the magnetic flux turns through 90" and is at some point in a direction which is not the rolling direction. This consequently produces an increase in total power loss.

Fig. 3 shows a graph of the variation of power loss, in an uncoated sample taken from the joint region of a transformer, with the angle between the rolling direction (direction of easy magnetisation) and the actual direction of magnetisation for the three cases when the sample is put under tensile stress, no stress and compressive stress in the direction of magnetisation.

Fig. 4 shows a similar graph to that of Fig.

3 but in this case the sample comes from a core in which the laminations have been coated with a tensile insulating coating in the normal way by the manufacturers of the grainorientated electromagnetic sheet steel. In practice transformers may be operated at 1 > 1.8T.

While in Figs. 3 and 4 loss data has been presented at 1.3T, 50Hz, the observed relationships hold at higher levels of induction.

As can be seen, in each case, the power loss when the direction of magnetisation is approximately parallel to the rolling direction, i.e. in the elongate regions of the limbs and yokes, is at a minimum when tensile stress is applied to the sample. This is, of course, to be expected and is the reason that the manufactuerers apply a tensile coating to the steel in the first place.

However, it will also be clear from Figs. 3 and 4 that when the direction of magnetisation is at 40-50 to the direction of rolling, i.e. in the joint regions, the minimum power loss occurs when the strip is under compressive strain. Even when the strip is under no stress, the power loss is considerably reduced below the power loss value obtained under the action of tensile stress.

Accordingly, the invention provides a laminated core for a transformer in which at least one of the laminations has a tensile coating only along the part of the yokes and limbs which do not form part of the joint regions.

Usually the core will be formed from a set of laminations cut from a roll of grain-orientated electromagnetic steel which has a tensile insulating coating applied to it, the laminations having this tensile coating removed in the parts which will form the joint regions in the completed core.

In one embodiment, the laminations may have an insulating coating providing a compressive strain applied to them in the joint regions. Preferably, however, the joint regions of the laminations, either in the uncoated condition or the conventional condition of possessing a tensile coating, are scribed at 45" to the rolling direction as described in our copending application No. 8529485 (TF/2465) which should be referred to for further information. This scribing is preferably produced by a laser beam. Such scribing is equivalent to the application of a compressive strain in the rolling direction of the sheet steel.

The invention will now be described by way of example with reference to Fig. 5 of the drawings of which: Figure 1 shows the conventional method of manufacturing a core for a transformer; Figure 2 shows the magnetic structure of grain-orientated electromagnetic sheet steel; Figure 3 shows a graph of the variation of power loss versus the angle between the direction of magnetisation and the rolling direction for an uncoated sample under tensile stress and compressive stress; Figure 4 shows a graph similar to that of Fig. 3 but for a sample already coated with a tensile insulating coating; and Figure 5 shows a lamination for use in a core according to the invention.

Referring then to Fig. 5, a lamination 15 for a 5 limb transformer is made of grain-orientated electromagnetic sheet steel which has been coated with a tensile insulation coating.

The lamination 15 is formed of several longitudinal pieces of this steel so that the limbs 16 and the yokes 17 have the direction of easy magnetisation (the rolling direction) along their longitudinal extent. The joint regions-that is the corner joints 18 and the Tjoints 19 are areas where the direction of magnetisation in the core are not along the direction of rolling. As explained above with reference to Figs. 3 and 4, in this case the tensile coating produces an increased power loss in this region. Thus in accordance with the invention, the tensile coating is removed in these joint regions 18 and 19.

In order to further reduce the power loss in these regions, they are scribed using a laser beam (as shown on Fig. 5) at 45O to the direction of rolling. This produces an increase in the number of closure domains and thus provides a decrease in power loss. More information on this can be obtained from our copending patent application No. 8529485 (TF/2465).

Claims (8)

1. A laminated core for a transformer in which at least one of the laminations has a tensile coating which is effective only along part of the yokes and limbs which do not form part of the joint regions.

2. A laminated core for a transformer according to Claim 1, wherein the core is formed from a set of laminations cut from a roll of grain-orientated electromagnetic steel which has a tensile insulating coating applied to it, and the laminations have the said tensile coating removed in the parts which will form the joint regions in the completed core.

3. A laminated core for a transformer according to Claims 1 or 2, wherein the laminations have an insulating coating providing a compressive strain applied to them in the joint regions.

4. A laminated core for a transformer according to Claim 2, wherein the uncoated joint regions are scribed at substantially 45O to the rolling direction of the respective lamination.

5. A laminated core for a transformer according to Claim 1, wherein the core is formed from a set of laminations cut from a roll of grain orientated electromagnetic steel which has a tensile insulating coating applied to it, and the joint regions of the laminations are scribed at substantially 45O to the rolling direction of the respective laminations so that the tensile coating is effective only over remaining regions of the laminations.

6. A laminated core for a transformer according to Claim 4 or 5, wherein the scribing is produced by a laser beam.

7. A laminated core for a transformer substantially as herein described with reference to Figs. 1 to 5 of the accompanying drawings.

8. A transformer containing a laminated core according to any preceding Claim.